1. Field of the Invention
The present invention relates to stepper motors and, more particularly, is directed towards a method and apparatus for critically damping the natural resonance of a stepper motor.
2. Description of the Prior Art
Stepper motor drive systems energize the field windings of a stepper motor in a step-by-step sequence which causes the armature of the motor to rotate in a corresponding step-by-step manner. Such systems have been designed for full step and fractional step operation, the smaller steps providing finer resolution. U.S. Pat. No. 4,042,868 discloses a stepper motor control circuit which utilizes a duty cycle converter for converting an input analog signal to a velocity squared feedback signal. U.S. Pat. No. 3,628,119 shows a stepper motor control system in which feedback signals are logically combined with acceleration and deceleration pulses. U.S. Pat. No. 4,095,157 teaches the use of a bi-directional pulse generator and up/down counters for controlling a stepper motor. U.S. Pat. Nos. 3,924,721; 4,147,968 and 4,151,448 disclose stepper motor control system utilizing feedback and command signals in conjunction with comparators. U.S. Pat. No. 3,868,557 shows a stepper motor damping apparatus in which a tachometer signal is converted to a binary signal. U.S. Pat. No. 4,140,956 discloses a stepper motor control circuit in which stepper motor drive signals are developed from multi-step function position command signals. U.S. Pat. No. 4,091,316, which is assigned to the assignee of the present invention, discloses a stepper motor drive system in which tachometer feedback is used to modulate the phase angle of stepper motor command currents for damping the basic stepper motor resonance.
Generally, stepper motor drive systems suffer from the disadvantages of vibration and loss of synchronism. Vibration occurs when the step rate is matched to motor resonance and when a cyclic current waveshape matches motor resonance. Analysis of tachometer signals indicates that loss of synchronism is due to vibration which starts at the motor shaft frequency and slowly builds in amplitude until the system is eventually knocked out of synchronism. Each of these disadvantages are related to extremely underdamped mechanical shaft resonance.